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Summary

All shelled creatures from the sea shore with the exception of barnacles, which are arthropods. Molluscs
are abundant in the region between high and low tide and are a major feature of all rocky ocean shores.
Molluscs make up a one of the animal kingdom's largest groups, with over 79,999 known species.

Most molluscs have these 9 standard body parts, the mantle(Shell), tentacle, gill, foot and the mouth. It is believed that modern molluscs evolved from a creature similar to the one above. It had its gills, reproductive organs and excretory organs within its mantle cavity at the rear of the animal.

Gastropoda

Gastropoda refers to a sub-group within Mollusca and means ‘stomach-foot’ in Latin. Gastropods are a collective group of ‘slugs and snails’ and are the most well-known of the sections that make up the Mollusca family. Gastropods inhabit many diverse landscapes across the earth, from gardens to deserts to the oceans! There are 611 families of gastropods (and 202 of these are extinct as they have only been found as fossils)! It is estimated that there are 85,000 species of gastropods in the world, making it easily the biggest sub-group within Mollusca. Gastropods with shells are commonly referred to as snails and ones without as slugs. They have digestive systems, circulatory systems, respiratory systems, excretory systems and reproductive systems which may differ from species to species to suit their environment (and certainly differ from ours)! For example, most marine gastropods breathe with a gill, while many freshwater and terrestrial (lives on land) gastropods use a ‘pallial lung’. Those with a lung are known as ‘Pulmonata’ and those with gills are ‘Paraphyletic’. Shells are important for many members of gastropoda, as it provides a certain amount protection from predators. Most are one-piece, spiral secretions and many have an opening trapdoor at the entrance to the shell called an ‘operculum’. Gastropods have some basic sensory organs and nervous systems, including eyes! Gastropods have no ears (and as a result, cannot hear)! However, some gastropods’ eyes can only distinguish between light and dark, whilst others have developed lens eyes! They do actually have two nervous systems, one being the ‘peripheral nervous system’, the other being the ‘central nervous system’. Gastropoda is such a large group and its species stretch so far over to world that the feeding habits of gastropods differ dramatically. Some feed off detritus, other are predatory carnivores, some scavengers, some herbivores and some are even parasites. In terms of diversity, Gastropoda is the second greatest group, beaten only by the Insects!